A material is generally described as being workable if it can be cold worked and hot worked. Nickel materials are usually hot workable. A nickel material which is sufficiently cold workable can be produced by a method which is based on the conventional processes of ferrous metallurgy and in which the molten material is degasified. This is accomplished by the known boiling reaction, which results from the formation of carbon monoxide in the molten material. Depending on the initial analysis of the molten material, the formation of CO is caused by an addition of carbon or of oxygen, the latter mainly in the form of metal oxides. This step enables also an adjustment of a predetermined carbon or oxygen content.
Because the reaction,
C +niO - CO +Ni
As an equilibrium reaction never results in a quantitative formation of CO, an undesirably high residual content of oxygen will always remain in the molten material, depending on various parameters, such as temperature and the slagging. This residual oxygen must be removed as completely as possible by an addition of elements which have an affinity to oxygen, such as magnesium, aluminum, manganese, silicon and in some cases also lithium and titanium. Because this oxygen-removal technique also involves equilibrium reactions, a surplus of the elements having an affinity for oxygen must be added if the deoxidation is to be as complete as possible. Whereas in a deoxidation with carbon the reaction product escapes as a gas, a considerable part of the reaction products formed by the deoxidation with metals having an affinity for oxygen remains in the molten materials as exogenous oxides. These deoxidation residues may give rise to considerable difficulties during the further processing of the nickel materials and during the use thereof by the final consumer.
For instance, elevated temperatures during the final processing or during the subsequent use of the nickel material may cause metallic residues of deoxidizers to diffuse out so that solderability or, in parts used in vacuum technology, the production of high vacuums will be highly adversely affected. Such metallic residues of deoxidizers will also be disturbing if the part is to be sealed in fused glass or is to be enamelled, mainly because such metallic residues of deoxidizers in contact with elements having high vapor pressures (Pb, Zn) result in an undesirably high porosity. Such elements are present in most of the contacting materials referred to. Similar remarks are applicable to the residual carbon which is still contained in nickel materials made by known processes and which reacts with such elements to form CO so that blowholes are produced in the surface layer.
Regardless of the size of the ingot, nickel materials made in the conventional manner from molten materials which have been deoxidized have the further disadvantage that certain exogenous oxides deposit at the grain boundaries and may give rise to stress crack corrosion in highly corroding environments under high mechanical stresses, particularly in contact with caustic alkaline solutions at elevated temperatures.